I’m generating flow text,

on a single line, is a style around an arbitrary text selection possible?

can I change letter_spacing on a single phrase in a line?

It's not possible to pass a text selection to the extension system ( i.e: text that you have highlighted using the mouse ). Just the parent text object.

I checked in chat. 

You can of course wrap any text you like in a <tspan> then style the tspan. 

I don't have much experience with svg text. So I can't really advise on kerning etc.

Don't forget that you can always use xpath to examine text elements and tspans etc.

I didn’t know about <tspan>, I will dig into that. Thanks for those tips. 

I didn’t know about <tspan>, I will dig into that. Thanks for those tips. 

When I get stuck I sometimes have the .svg file open in Geany, or Notepad++.

Change something then save. Then look at the changes in the .svg. 

I think flowed text makes extensive use of <tspan>.

I also wrote an extension - which I did not post on inkscape.org, because I didn't like it  - https://gitlab.com/inklinea/textractor

Code sample to get you started:

txt = inkex.TextElement(x='0', y='100')
txt.set('xml:space', 'preserve')
txt.text = 'This text is '
txt.style = 'font-family: serif; font-size: 36pt'
tspan1 = inkex.Tspan()
tspan1.text = 'bold'
tspan1.style = 'font-weight: bold'
txt.append(tspan1)
tspan2 = inkex.Tspan()
tspan2.text = '!!!'
txt.append(tspan2)

Thanks for that code example -- very helpful for hacking.

Is there a newline command?

I've been working with Flowtext, but it's overkill for what I need. 
I'm a little confused about how I'm even successfully doing newlines there.

DrQt

Is there a newline command?

Not as such.  Instead, you pass coordinates to inkex.Tspan as follows:

txt = inkex.TextElement(x='0', y='100')
txt.set('xml:space', 'preserve')
txt.text = 'This text is '
txt.style = 'font-family: serif; font-size: 36pt'
tspan1 = inkex.Tspan()
tspan1.text = 'bold'
tspan1.style = 'font-weight: bold'
txt.append(tspan1)
tspan2 = inkex.Tspan(x='0', y='160')
tspan2.text = 'and split across lines.'
txt.append(tspan2)

That is, you append Tspans to a Text object for both style changes and line breaks.

Ok!  Thank you for that clarification.